With the increasing proliferation of digital TV and radio systems, metadata, i.e. “data about data”, intended to guide receiver-side control of program loudness and dynamic range as well as controlling stereo downmix is taking on increasing importance. Because the emission format or the transmission format of audio data is not necessarily the same as the production or the contribution format, i.e. the encoding scheme used during the creation or the post-processing of the audio signal, means must be found to transcode the metadata from one format to another. With the introduction of the audio codec HE AAC, which is also known as aacPlus, into several new broadcasting systems, the topic of how to best encode new metadata and how to transcode pre-existing metadata into and from HE AAC needs to be addressed.
The HE AAC (High Efficiency Advanced Audio Coding) is a state-of-the-Art low-bitrate codec, providing full bandwidth, near CD audio quality at 48 kbps stereo. It has been standardized in MPEG-4 under the HE AAC profile. The profile offers enhanced ways on the optional application of metadata in the decoder in a way that improves upon a number of shortcomings present in the original MPEG-2 AAC definition. In particular, it allows the specification of metadata such as DRC (Dynamic Range Control), DN (Dialog normalization), or downmixing from multichannel to stereo, which is widely used in broadcasting to achieve adequate reproduction of the original program material in particular listening environments.
While codecs such as HE AAC are broadly used in broadcasting, different coding schemes are common on the production side as well as on the receiver side. By way of example, the Dolby E coding scheme is frequently used by content producers, whereas the Dolby Digital coding scheme, which is also referred to as the AC-3 or the Dolby D coding scheme, is frequently used in Theater and Home Theater decoding systems. Consequently, there is a need for the transcoding of audio metadata between the different coding schemes which ensures that metadata which was originally defined at the producing side reaches the final decoding system in a non-deteriorated way. The target is in general to ensure that no or little perceptual differences can be detected by the auditor when comparing the originally encoded audio stream subjected to the originally defined metadata with the received audio stream subjected to the transcoded metadata. The following invention deals with this problem and defines methods for translating metadata from one compressed bitstream format to a different compressed bitstream format, particularly in cases where the framing of the two formats is not compatible.